1. Field of the Invention
The invention relates to image processing method and apparatus for binarizing input multi-level image data into binary data.
2. Related Background Art
Hitherto, in an image processing apparatus of a digital copying apparatus, a digital facsimile, a printer, or the like, input multi-level image data is binarized into binary data.
Hitherto, in a case of converting multi-level data of a low pixel density into binary data of a high pixel density and when a importance is attached to a processing speed, a method of binarizing while executing a high pixel density realizing (enlarging) process, such as a density pattern method, is used. In a case of attaching an importance to a picture quality, a binarizing method whereby after multi-level data has been enlarged, binary data is also regarded as one pixel for one pixel of the multi-level data, such as an error diffusion method, is used.
As inventions regarding the error diffusion method, the same applicant as the present invention has filed U.S. Pat. Nos. 4,876,610, 4,878,125, 5,008,950, 4,958,236, 4,975,786, 4,958,238, and 5,086,484.
However, according to the above conventional examples, when the multi-level data of a low pixel density is converted into the binary data of a high pixel density, in a case of binarizing by using the binarizing method like a density pattern method whereby a plurality of pixels of the binary data are allocated to one pixel of the multi-level data, the binarization can be performed at a high speed. However, errors which are generated upon binarization cannot be propagated and a limitation occurs in the gradation expression, so that there is also a problem in terms of the picture quality. On the other hand, in a case of performing the binarization by using a binarizing method like an error diffusion method whereby one pixel of the binary data is allocated to one pixel of the multi-level data and errors generated upon binarization are diffused, thereby obtaining an excellent gradation expression, the binarization can be performed with a high picture quality. However, since many processes such as an error diffusing process and the like which need a long time are executed, it takes a long time for binarization. In addition, since an enlarging process (resolution converting process) must be also performed, there is also a drawback such that it takes a long processing time.
In a case of performing the binarization by using the error diffusion method, since the positions at which the dots of Y, M, and C are printed are unstable, there is a drawback such that the dots of YMC to be printed inherently at the same position are not always printed to the same position, so that a binarizing method which is not suitable to the subtractive color mixture theory is executed and color reproducibility is deteriorated.